In pipelines for long-distance conveyance of natural gas, crude oil, and the like, conveyance efficiency is improved through increasing running pressure, whereby conveyance cost is reduced. In order to increase running pressure, the wall thickness of pipe must be increased, or the strength of pipe material must be increased. However, the increase of the wall thickness of pipe involves impairment of field weldability and the necessity of enhancing the foundation structure to cope with an increase in pipeline weight. Under these circumstances, there have been increasing needs for enhancing the strength of welded steel pipes. For example, recently, American Petroleum Institute (API) has standardized X80 grade welded steel pipes having a yield strength (YS) of not less than 551 MPa and a TS of not less than 620 MPa, and put them into practical use.
As a result of enhancement of the strength of welded steel pipes, the manufacture of welded steel pipes of up to X100 grade (YS: not less than 689 MPa; TS: not less than 760 MPa) based on the technique for manufacturing X80 grade welded steel pipes is known to be feasible. Furthermore, there has been proposed high-tensile-strength steel having excellent low-temperature toughness and field weldability and a TS of not less than 950 MPa (Japanese Patent Application Laid-Open (kokai) Nos. 8-104922 and 8-209291).
As far as low-temperature toughness and resistance to cold weld cracking at a relatively small heat input are concerned, the manufacture of steel products used for welded high-strength steel pipes is feasible through the above-mentioned technical development. However, the manufacture of high-strength welded steel pipes requires not only the above-mentioned high-tensile-strength steel but also high-strength weld metal having appropriate toughness. It has been known that the toughness of weld metal is improved through refinement of microstructure. Specifically, there emerges a wide practical use of a weld metal in which fine "acicular ferrite" is formed by adjusting the Al/O (oxygen) value through addition of trace Ti and B into the weld metal. However, generally, strength attained by acicular ferrite is limited. Acicular ferrite in weld metal cannot stably provide a TS of not less than 900 MPa. Accordingly, in order to obtain a TS of 900 MPa while appropriate toughness is provided, another method must be employed. Particularly, when welding heat input is increased in order to improve efficiency of welding, the cooling rate of weld metal decreases. Accordingly, a TS of not less than 900 MPa becomes difficult to attain.